Recently, introduction of a 4th generation cellular system (4G) has been under discussion in order to achieve improvement of additional performance of wireless communication. In 4G, techniques such as a relay technique, a carrier aggregation, and a coordinated multiple point transmission and reception (CoMP) are attracting attention.
The relay technique refers to a technique by which a relay node relays communication between a base station (for example, a macro cell base station) and a communication terminal, and the relay technique is important in improving the throughput at a cell edge of a base station. The carrier aggregation is a technique in which a use bandwidth is increased and a maximum throughput is improved by collectively treating a plurality of frequency bands each having a bandwidth of 20 MHz. CoMP is a technique by which a plurality of base stations collaborate with each other to perform data communication with a communication terminal, and the coverage of high-data-rate communication can be increased.
In further detail, CoMP refers to a technique by which a plurality of base stations simultaneously perform data communication with one communication terminal. According to this technique, since branches of a plurality of base stations can be used for data communication, an antenna gain and a signal to interference plus noise ratio (SINR) can be improved.
Here, it is assumed that a base station has calculated a weight of a branch based on a channel matrix of an uplink from a communication terminal and then has obtained reception directivity. In this case, when a weight of a branch is used even at the time of transmission, transmission directivity matching reception directivity is considered to be obtained. However, since a transfer function of a transmission analog unit and a transfer function of a reception analog unit of each branch are different from each other, actual transmission directivity does not match reception directivity.
Problems caused due to the difference between the transfer function of the transmission analog unit and the transfer function of the reception analog unit of each branch are solved such that each base station individually performs branch calibration and a calibration coefficient of each branch. The calibration coefficient is a coefficient used to regularize a ratio of the transfer function of the transmission analog unit and the transfer function of the reception analog unit of each branch in one base station. The branch calibration is disclosed in, for example, Patent Literature 1 and Patent Literature 2.